The application of polymers derived from renewable natural resources as the initial material in various products is becoming increasingly common and important. Such products are biodegradable and thus do not cause problems even as waste. Awareness of environmental problems has made the use of native polysaccharides and their derivatives highly desirable. Thus, it would be desirable if biodegradable polysaccharides from nature or derivatives thereof could be used as a partial raw material for preparing adhesives and coatings for different active ingredients, such as pharmaceuticals and fertilizers.
Green thinking and emphasis of nature-friendliness are creating new markets for products based on renewable natural resources. Such trends are visible, for example, in the packaging industry, where recyclability, reuse, compostability, biodegradability are some of the requirements of today. Of course, the requirements are imposed on all components of packaging, also on hot melt adhesives, one of the principal areas of use of hot melt adhesives being the packaging industry. Corresponding trends are observable also in other major areas of use of hot melt adhesives, such as non-woven products (diapers, sanitary napkins, etc.), in which friendliness to the environment and non-hazardousness are emphasized. Also clearly visible is a trend to replace petrochemical products with biopolymer products.
The product of the present invention is a product suitable for use as a component in hot melt adhesives. Hot melt adhesives are often composed of three principal components: a thermoplastic polymer, a tackifier and wax. In addition, small amounts of various additives are used for controlling the properties of the adhesive. The most common conventional tackifiers used in hot melt adhesives are derivatives of tall oil resin or hydrocarbon resins. The adhesives also contain wafers. The most commonly used waxes are synthetic PE waxes. In other words, the major proportion of the raw materials contained in hot melt adhesives are products from the petrochemical industry.
The use of polymers derived from renewable natural resources, especially starch and hemicellulose compounds, as the initial material in various applications has become increasingly common. These new products are usually more or less biodegradable and thus do not cause problems even as waste. Awareness of environmental problems has made the use of polysaccharides from nature and their semisynthetic derivatives especially desirable. At the same time the demand for development of new more advantageous, modified methods has become increasingly important and timely. It is desirable that these new methods can be used with a good yield and that the end product can be isolated easily. It is desirable that the solvents and byproducts can be recovered and reused without difficulties.
The methods of preparing fatty acid esters of polysaccharides are generally known and constitute knowledge belonging to basic chemistry. These methods can, however, be modified and optimized in order to produce as easily and economically as possible the most suitable compounds for the specific applications.
The state-of-the-art methods do not fulfill the requirements of the method described in the present invention. The degree of substitution of the products is usually too low. Therefore the products are not sufficiently hydrophobic. Even if their degree of substitution were higher, their permeability to water would probably be too high for our applications. The preparation methods are complicated, slow and hazardous, and the isolation of the product is cumbersome. The solvents used in the methods are either toxic or carcinogenic, or their handling is otherwise objectionable.
For example, U.S. Pat. No. 3,666,492 describes C.sub.8 -C.sub.14 fatty acid esters of hydrophobic starches, suitable for use in chewing gums and having a degree of substitution (DS) of at least 2, and a method for their preparation. Dioxane is not a desirable solvent because of its toxicity.
By the use of the method of our invention, described below, most of the disadvantages described above are eliminated. When dialkyl formamide is used as a solvent in a reaction between polysaccharides, in particular starch and fatty acid chlorides, a highly substituted fatty acid ester of polysaccharide is obtained. These highly substituted products are especially suitable for use as components in hot melt adhesives and as coatings for active agents particularly fertilizers.
The reaction of the present invention takes place in more advantageous conditions, the isolation of the product is easy, and the product is obtained with a very good yield. We have also observed that both hexosans and pentosans can be modified in the same manner.
Thus the invention provides an economic method to produce with a good yield highly substituted fatty acid esters of the polysacchariides, which melt at low temperatures and which are poorly permeable to water.
A further objective of the invention is to replace previously used methods by avoiding the expensive and carcinogenic dioxane and by replacing it with a more economical, less toxic and less hazardous solvent. E.g. dioxane may readily form explosive peroxides and its self-ignition temperature is 180.degree. C. This is avoided by using our method. Furthermore, the reaction of our invention can, owing to the change of solvent, be performed at a higher temperature, thus shortening the reaction period and enabling a higher amount of product to be obtained per reactor.
Still another objective of the invention is to provide a method in which the recovery of the spent reagents and solvents is easy.
The highly substituted, low-melting, poorly water-permeable fatty acid esters of the polysaccharide of the present invention are especially suitable for use in hot melt adhesives and in coatings for fertilizers. Being products based on renewable natural resources, they enable the petrochemical-based polymers commonly used in the said applications to be replaced with semisynthetic biopolymers.